This invention relates to a vacuum valve (a vacuum circuit breaker), and, more particularly, to an alloy material which can be used as contacts in the vacuum valve.
Principal characteristics required for a contact forming material for a vacuum valve are welding-resistance, voltage withstanding capability, and current interrupting property. Important requirements other than these fundamental requirements are low and stable temperature rise and low and stable contact resistance. However, these requirements contradict each other and therefore it is impossible to meet all of the requirements by a single metal. Accordingly, in many alloy material which have been practically used, at least two elements which compensate mutually inadequate performance thereof have been used in combination to develop alloy materials which are suitable for specific uses at a large current, at a high voltage or at other conditions. Alloy materials having excellent characteristics have been developed. However, demands for a contact forming material for a vacuum valve which withstands higher voltage and larger current have increased, and a contact forming material for the vacuum valve which entirely meets such requirements has not been obtained.
For example, Japanese Patent Publication No. 12131/1966 discloses a Cu-Bi alloy containing no more than 5% of an anti-welding component such as Bi. This reference describes that the Cu-Bi alloy can be used as a contact forming material which is used at a large current. However, the solubility of Bi in the Cu matrix is extremely low, and therefore segregation occurs. Further, the surface roughening after current interruption is large and it is difficult to carry out processing or forming. Japanese Patent Publication No. 23751/1969 discloses the use of a Cu-Te alloy as a contact forming material which is used at a large current.
While this alloy alleviates the problems associated with the Cu-Bi alloy, it is more sensitive to an atmosphere as compared with the Cu-Bi alloy. Accordingly, the Cu-Te alloy lacks the stability of contact resistance or the like. Furthermore, although both the contacts formed from the Cu-Te alloy and those from the Cu-Bi alloy have excellent anti-welding properties in common and can be used sufficiently in prior art moderate voltage fields in respect to voltage withstanding capability, it has turned out that they are not necessarily satisfactory in applying to higher voltage fields.
On the other hand, a known contact forming material which is used at a high voltage is a sintered alloy of Cr and a highly conductive component such as Cu (or Ag). However, Cr is a metal which is extremely readily oxidized and therefore, of course, the management of Cr powder or its compact is important. Atmospheres which are used during preliminary sintering and during infiltration affect the characteristics of the material. For example, in the practical manufacturing process, even in the case of the Cu-Cr alloy obtained by thoroughly controlling the preliminary sintering temperature and time, and the infiltration temperature and time, the contact resistance of temperature rise characteristics vary and are unstable. Contacts having stability without scattering are required.